1) Field of the Invention
The present invention relates to a microwave oven, and more particularly, to a waveguide for a microwave oven that can enhance the heating uniformity.
2) Description of Related Art
Generally, a very high frequency (VHF) required for apparatus, such as a broadcasting apparatus, a hair dryer, and a microwave oven, which uses electromagnetic wave is generated by a magnetron.
Such a magnetron generates through an antenna very high frequency energy of about 2.45 GHz when an acceleration voltage of about 4.2 KV is applied to the magnetron.
To apply the magnetron device to, for example, a microwave oven and to effectively cook a variety of food loads using the VHF energy, a guide system such as a waveguide is used to direct the VHF energy generated by the magnetron to a cooking chamber of the microwave oven.
The antenna of the magnetron device generates linear polarization energy which is radiated into the cooking chamber through a waveguide to heat the food load. FIGS. 1a and 1b show a conventional electromagnetic wave guide system employed in a microwave oven.
The conventional electromagnetic wave guide system comprises a waveguide 11, mounted on a sidewall of a cavity body 12 defining a cooking chamber 130, for guiding electromagnetic wave energy into the cooking chamber 130 through an opening 11b; a magnetron 10, mounted on a side of the waveguide 11, for generating the electromagnetic wave energy and radiating the same into the waveguide 11 through an antenna 10a projected into the waveguide 11 when a high acceleration voltage is applied from a high voltage transformer (not shown); a projection 11a, formed between the outer side of the waveguide tube 11 and the magnetron 10 to provide a short circuit having a length equal to that of the antenna 10a of the magnetron 10; and an electromagnetic wave inlet port 12a through which the electromagnetic wave energy radiated through the opening 11b of the waveguide 11 is directed into the cooking chamber 130, the electromagnetic wave inlet port 12a being formed on the sidewall of the cavity body 10.
When the magnetron 10 is excited by the high acceleration voltage generated by the high voltage transformer, the magnetron 10 generates electromagnetic wave energy of, for example, about 2.45 GHz, and at the same time, a turn table 14, disposed on a bottom of the cooking chamber 130 and on which food 14 is located, starts rotating.
The electromagnetic wave energy generated by the magnetron 10 is radiated into the waveguide 11 through the antenna 10a, then converted into a standing wave at the projection 11a forming the short circuit, and finally radiated into the cooking chamber 13 through the opening 11b and the inlet port 12a of the cavity body 12 to heat the food 15.
In the above, the electromagnetic wave energy is radiated from the antenna 10a in the form of a wave motion.
Accordingly, electromagnetic wave reflected on the projection 11a forming the short circuit surface is added to electromagnetic wave radiated from the antenna 10a at the opening 11b of the waveguide 11, thereby forming the standing wave.
In addition, the electromagnetic energy heats the food 15 within the cooking chamber 130 when low electric field is combined with high electric field.
However, in the above described electromagnetic wave system, the projection 11a formed to easily form the standing wave makes the structure of the waveguide 11 complicated. Furthermore, since a contacting area between the short circuit of the waveguide and the antenna is too small, the electromagnetic wave energy is not sufficiently radiated from the antenna, deteriorating the cooking performance.
In addition, the electromagnetic wave radiated from the antenna is inherently straight polarization having a constant polarization section with respect to its advancing direction, degrading the cooking uniformity due to an electromagnet wave interference effect occurring within the cooking chamber 130.
In the straight polarization, a polarization section of the electric filed direction in one direction is constant. That is, due to the electromagnetic wave interference, the electromagnetic wave is divided into a hot point and a weak point, by which the improvement of the cooking performance is restricted.
However, if circular polarization having a polarization section which rotates with respect to an advancing direction of the electromagnetic wave as the time goes is formed, since a direction of the electric field is continuously changed as the time goes, a reflecting angle of the electromagnetic wave transmitted to the cooking chamber 130 is also continuously changed, thereby dispersing the electromagnetic wave to a wider area to improve the cooking uniformity.
Therefore, in recent years, waveguides which can convert the electromagnetic wave energy generated from an antenna of a magnetron into the circular polarization to improve the cooking uniformity have been developed.
FIG. 2 shows a conventional circular polarization generating system which can be also applied to a microwave oven.
The conventional polarization generating system is a four port hybrid junction, comprising a rectangular waveguide 11, an electromagnetic wave source such as a magnetron is coupled on an end of the waveguide 11, a short circuit 17 disposed on the other end of the waveguide 11.
A polarization radiator 12, which is an opening or a pair of slots, is disposed on a bottom of the waveguide 11 to radiate electromagnetic wave.
In addition, electromagnetic wave energy radiated into another section of an electromagnetic wave guide 20 has left-hand circular polarization or right-hand circular polarization.
The radiator has two ports as the left-hand circular polarization and the right-hand circular polarization that are insulated from each other.
A phase shifter 19 is disposed on the short circuit 17 to shift a phase of the electromagnetic wave radiated from the radiator.
Here, when assuming that the phase shifter 19 is not disposed on the short circuit 17, electromagnetic wave energy generated from the antenna of the magnetron and transmitted to a source port 1 is dispensed to ports 4 and 2. A portion t1 of the dispensed electromagnetic wave energy a1 has the right-hand circular polarization and is radiated toward the cooking chamber through the radiator, and another portion b1 of the dispensed electromagnetic wave energy a1 is reflected after passing through the radiator 12.
Electromagnetic wave energy reflected from the port 2 is divided at the ports 1 and 2 and a portion t2 of the reflected electromagnetic wave energy a2 have the left-hand circular polarization and are radiated into the cooking chamber by the radiator. Another portion b2 of the reflected electromagnetic wave energy a2 passes through the radiator.
Since the two left-hand circular polarizations as well as the two right-hand circular polarizations have a different polarity, the formal left and right-hand circular polarizations always exist at the electromagnetic guide 20.
Therefore, the right-hand circular polarizations and the left-hand circular polarizations collide against each other, thereby forming a standing wave.
Here, if the electromagnetic wave guide 20 is a cavity body of the microwave oven, the right-hand circular polarization reflected from food is converted into the left-hand circular polarization.
A part of the reflected energy passing through the radiator is intensively coupled on the port 2 but weakly coupled on the port 1. Therefore, the electromagnetic wave energy directed to the port 2 is reflected again and converted into the left-hand circular polarization.
Therefore, the radiated electromagnetic wave energy is formed as the standing wave with the right and left-hand circular polarizations at the electromagnetic wave guide 20.
To rotate the standing wave, it is required to change one phase of the two polarizations. The phase shifter 19 disposed on a front side of the short circuit 17 of the waveguide 11 converts a phase of the left-hand circular polarization of the reflected electromagnetic wave energy, thereby rotating the standing wave. The rotating standing wave functions as a mechanical rotating opening.
The energy radiated as a rotating wave improves the heating uniformity within the cooking chamber.
In the above described circular polarization generating system, the mounting of the phase shifter and the radiator within the waveguide makes the length of the waveguide much longer and the structure thereof complicated, increasing the manufacturing costs. In addition, the long waveguide makes it difficult to dispose other electric parts within an electric part mounting chamber. Therefore, the electric part mounting chamber should be enlarged.
In an effort to solve the above described problems, a guide partition is mounted within the waveguide in the longitudinal direction to divide the interior of the waveguide into two parts, and a pair of rectangular openings are formed at an angle of 45.degree. on both sides of the guide partition.
However, the guide partition additionally provided makes the system further complicated, and it is very difficult to precisely form the openings at the angle of 45.degree..